The advantage of airborne electromagnetic surveying systems is that a greater amount of surface area can be covered when conducting geological surveying for mineral exploration. In conducting airborne electromagnetic surveying, usually an airborne vehicle is fitted with a transmitter, which can be mounted on or towed by the airborne vehicle, such as a helicopter, airplane or other aircraft, for emitting a primary electromagnetic field for surveying terrain over which the airborne vehicle is flying. A receiver then receives and records a resultant field, corresponding to the interaction of the primary field with the underlying terrain, and which comprises a combination of the primary electromagnetic field emitted by the transmitter as well as a secondary field emanating from the underlying terrain. This secondary field may then be processed, after it is received, in order to ascertain the nature and geological composition of the underlying terrain.
Because the secondary field emanating from the underlying terrain is generally much smaller in amplitude than the primary electromagnetic field, the primary electromagnetic field can overwhelm the receiver and interfere with its ability to sense the secondary field. Further, such transmitted electromagnetic fields generally generate eddy currents not only in the Earth but also in the proximate metallic parts including those of the system itself and the aircraft body. The secondary fields of these eddy currents constitute noise, which can adversely impact the survey data and generally increase the difficulty in obtaining reliable geological information from this data.
One of the most common ways to minimize this noise is by isolating the receiver as much as possible from the primary electromagnetic field emitted by the transmitter. Previously, such isolation was achieved by physically separating the receiver from the transmitter by as great a distance as possible. In general, the greater the distance between the receiver and the transmitter, the smaller the amplitude of the primary electromagnetic field at the receiver, and, accordingly, the lesser the interference with the receiver in detecting the secondary field.
Typically such distances are maintained between the receiver and the transmitter, by causing the receiver to be housed in a “bird” towed by the airborne vehicle.
However, separating the transmitter and receiver by housing the receiver in a bird leads to technical problems, with the receiver changing position relative to the transmitter, and detecting much of the primary field from the transmitter.
Another common means is to devise a transmitter loop structure containing the transmitter, to which is attached the separate receiver, in a rigid position as far away from the transmitter as possible, so as to maintain the distance therebetween as far as possible and the geometry therebetween as constant as possible.
However, there are a number of technical problems in designing such systems. First, such systems are generally larger and demand heavier frame constructions for carrying the transmitter and receiver. For example, due to the separation required between the transmitter and the receiver in the bird, it is not unusual for such devices to exceed 20 feet in length and up to several hundred pounds in weight. While such frames provide a certain amount of rigidity, which can provide less noise at the receiver, the heavier frame makes transportation of the bird difficult. The production costs and fuel costs associated with the manufacturing and use thereof can also be high.
In attempting to alleviate this problem, some prior art systems, such as that described in International Patent Publication No. WO 2004/046761 (Morrison et al, have utilized light weight support frame constructions, but these have tended to be overly flexible, as opposed to utilizing a rigid structure, and are thus susceptible to noise, through vibration during use.
It would therefore be advantageous to have a rigid transmitter loop structure for use in an airborne electromagnetic (EM) surveying system which maximizes the rigidity of the structure, so as to reduce vibratory noise, while, at the same time, minimizing the size and weight thereof.
It would be further advantageous to have an electromagnetic (EM) survey system that is capable of substantially completely cancelling the primary electromagnetic field signal emitted by the transmitter, while still measuring vertical and/or horizontal components of the resulting electromagnetic field.